A reactive transmit voltage may have a waveform such as a rectangular waveform or a pulsed signal sequence, depending on a required application.
The concepts described herein are also useful for the detection of conductors whether these are ferrous materials or non-ferrous materials but which are relatively good electrical conductors in environments containing relatively mildly conducting materials.
A general form of most metal detectors which are used to interrogate soils is a hand-held battery operated unit, a conveyor mounted unit, or a vehicle mounted unit.
Examples of hand-held products include detectors used to locate gold, explosive land-mines, coins and treasure. Examples of conveyor-mounted units include tramp metal or fine gold detectors in ore mining operations and industrial metal detectors to locate contaminants in food or pharmaceutical products, and examples of vehicle-mounted detectors include metal detector arrays used to locate explosive land-mines. These units usually consist of a transmit coil to transmit an alternating magnetic field associated with a reactive transmit voltage, transmit electronics which generate a transmit signal applied to the transmit coil, and receive electronics which receive a magnetic field and process received signals to produce an indicator output. By far the most numerous products of the above examples are the hand-held battery operated products. It is desirable that these have low power consumption to maximise battery life.
A transmit coil may be approximately represented as consisting of an effective inductive component impedance in series with an effective resistive component impedance which may include resistance of cabling and connectors and some elements of the transmit electronics.
One problem with metal detectors which transmit more than one frequency and are used to search an environment of varying magnetic permeability such as magnetic soils, is that the transmit coil's effective inductive component impedance is modulated by variable magnetic permeability, which in turn alters a reactive transmit voltage by differing amounts at different frequencies. Considering that a principal advantage of such metal detectors usually lies in an ability to compare a received magnetic signal interrogation of the environment at different frequencies (or frequency profiles), a different modulation of the transmit reactive voltage at different frequencies creates inaccuracies in useful received and processed signal calculations. In practice effective inductive component impedance may vary by up to a few percent in the most permeable soils.
Examples of multi-frequency transmission include simultaneous sinewaves, and all forms of “time domain” pulsed or rectangular waveforms. Pulsed or rectangular waveforms effectively transmit many frequencies as is known from Fourier analysis. Examples (which are not considered to be mere paper publications and are not acknowledged as being common general knowledge of multi-frequency transmission in magnetic soils) are given in U.S. Pat. No. 4,942,360 and examples of rectangular waveform transmission in magnetic soils are given in U.S. Pat. No. 5,537,041. The invention described herein may be advantageous over the art disclosed in these mere patent publications.
Accordingly a voltage applied to a transmit coil may be considered to have approximately two series voltage components. One results from an effective reactive transmit voltage (Vx) (non-energy dissipative, magnetic) across a transmit coil's effective inductive component impedance (X), and the other an effective resistive voltage component (Vr) (energy dissipative, non-magnetic) across a transmit coil's effective resistive component impedance (R). A vector sum of these two effective voltage components equals an applied voltage (Vapplied). That is, in terms of a sinewave of frequency w,Vapplied=Vr+Vx, whereVresistive=Vr=Vapplied(R/(sqrt(R2+X2))=Vapplied(R/sqrt(R2+(wL)2)  (i),Vreactive=Vx=Vapplied(X/(sqrt(R2+X2))=Vapplied(jwL/sqrt(R2+(wL)2)  (ii)
Where for a sinewave frequency w, X=jwL where L is the effective transmit coil inductance, and the total effective series impedance isZ=X+R=jwL+R  (iii)
An alternating magnetic field transmitted by the transmit coil is only related to the effective reactive transmit voltage component while the effective resistive transmit voltage component contributes nothing to this field. This transmitted magnetic field may induce both resistive and reactive magnetic fields in an environment owing to the properties of the environment which may in turn induce voltage signals in a receive coil used for magnetic reception. This induced voltage is applied to receive electronics for processing for assessment of the magnetically interrogated environment.
For two equal voltages applied to the transmit coil (Vapplied) at frequency w1 and w2, the ratio of the respective effective reactive transmitted voltage components at w1 and w2 equalsRatio12=sqrt(((R/w2)2+L2)/((R/w2)2+(w1/w2)2L2)  (iv)
To highlight that this is a function of L, this may be rewritten asRatio12=sqrt((k1+L2)/(k1+k2L2))  (v)
Where k1 and k2 are constants and the salient point is that k2 is not equal to 1.
In most multi-frequency transmission metal detector systems, the transmit/receive coil usually is a transmit coil plus a receive coil intrinsically nulled, but imperfectly so. Owing to both magnetic and capacitive coupling, the intrinsic coupling between the transmit and receive coil in practice is un-nulled and frequency dependent. The un-nulled components are usually temperature dependent.
Hence if the reactive transmit voltage varies differently at different frequencies with variations in L, one received component at one frequency may not be compared to another without knowledge of L, the properties of the transmit/receive coil null (and k1 and k2). This requires several different measurements and requires complex calculations.